In analytical instrumentation systems, such as clinical chemistry analyzers, liquid samples such as blood plasma or urine are often processed by separating each of a number of samples by a gas bubble, typically air, to create a stream of segments and flowing the stream of liquid and gas segments through an elongated tube. As the stream flows through the tube, various analytical measurements are performed. In some systems, the flow of the stream of liquid and gas segments is capable of reversing direction during processing.
In such systems, in order for the processing and analysis to be accurate, it is important to be able to discriminate between the liquid segments and the gas segments and to be able to determine the direction of flow of the stream. Devices which provide this functionality are known in the art and are typically called bubble detectors. One such prior art bubble detector is described in U.S. Pat. No. 5,466,946 to Kleinschmitt et al., hereinafter the '946 patent, the disclosure of which is incorporated herein by reference in its entirety.
FIGS. 1 and 2 are schematic cross-section diagrams of the above described prior art bubble detector described in the '946 patent. As shown in the FIGS. 1 and 2, the bubble detector includes an illumination fiber optic bundle 10 which receives white input light from light source 11 and directs the light through the diameter of a transparent tube 12 through which a liquid and gas segmented stream passes in a direction orthogonal to the longitudinal axis of the tube 12. Located on the side of the tube 12 opposite bundle 10 is a collection fiber optic bundle 14, which directs light to conventional analyzer instrumentation (not shown), and a smaller fiber optic bundle 15. Bundle 15 only receives light extending outside a predetermined zone in which bundle 14 is disposed. The light collected by bundle 15 is input into a photodiode 16 that converts the light to electrical energy. Comparator 17 compares the output of the photodiode 16 to a reference voltage 18 and generates a control signal 19.
As shown in FIG. 1, when the tube 12 contains liquid, the light from the illumination fiber optic bundle 10 is bent or refracted when passing through the liquid filled tube 12 so that substantially none of the light rays extend outside the predetermined zone and reach bundle 15. Under such circumstances, the signal from the photodiode 16 is low, i.e., below the reference voltage 18, so that the comparator 17 produces a control signal 19 having a first level which indicates that there is liquid in the tube 12.
As shown in FIG. 2, when the tube 12 contains a gas such as air, the light rays from the illumination fiber optic bundle 10 are not refracted or bent as severely, and consequently, a significant portion of the light is disposed beyond the predetermined zone and is collected by the bundle 15. As a result, the signal from the photodiode 16 is high, i.e., above the reference voltage 18, so that the comparator 17 produces a control signal 19 having a second level that indicates that a gas is in the tube 12.
The height of bundles 10, 14 and 15 and their proximity to the tube 12 must be chosen to assure sufficient light bending or refraction so that substantially none of the rays impinge upon bundle 15 when liquid is in the tube 12. The particular geometries are described in detail in the '946 patent and thus will not be repeated herein.
FIG. 3 shows another embodiment of the bubble detector described in the '946 patent wherein the direction of flow of a stream of liquid and gas segments is detected in addition to the detection of a liquid/gas interface. As shown in FIGS. 3 and 4, the bubble detector includes two light path channels made up of fiber optic bundles 15 and 15' that share the same illumination fiber bundle 10. The bundles 15 and 15' are spaced apart a distance less than the width of the smallest liquid or gas segment in order to detect the interface between a gas segment and a liquid segment.
The output from each of the bundles 15 and 15' is fed to photodiodes 16 and 16', the outputs of which are received by comparators 17 and 17' and compared to reference voltages 18 and 18' to produce outputs 19 and 19'. The outputs 19 and 19' are applied to logic circuit 20 which produces two output signals 21 and 22, one specifying the fact that a liquid/gas interface has occurred and the other specifying the direction of flow of the stream. The particulars of the logic circuit 20, and the method utilized therein to determine the direction of flow of the stream are described fully in the '946 patent and will not be repeated herein.
Because the bubble detector described in the '946 patent utilizes a single light source 11 and a single illumination fiber optic bundle 10 to direct light through tube 12 for each of the two light path channels, and because the light travels in substantially the same direction in each of the light path channels, there is a tendency for optical cross-talk to occur between the two channels. As a result, some of the light from the first channel may be received by the photodiode dedicated to the second channel and vice versa, thereby creating a signal discrimination problem.